Lubricating oils for the most part are based on petroleum fractions boiling above about 232 degrees C. (450 degrees F.). The molecular weight of the hydrocarbon constituents is high and these constituents display almost all conceivable structures and structure types depending in large part on the type of crude oil from which they were prepared.
The rational in lubricant refining is that a suitable crude oil, as shown by experience or by assay, contains a quantity of lubricant base stock having a predetermined set of properties such as, for example, appropriate viscosity, oxidation stability, and maintenance of fluidity at low temperatures. The refining process imposed to isolate that lubricant base stock currently consists of a set of subtractive unit operations which removes the unwanted components. The most important of these unit operations include distillation, solvent refining, and dewaxing, which basically, except for catalytic dewaxing, are physical separation processes in the sense that if all the separated fractions were recombined one would reconstitute the crude oil.
A lubricant base stock (i.e. a refined oil) may be used as such as a lubricant, or it may be blended with another lubricant base stock having somewhat different properties. The base stock, prior to use as a lubricant, may also be compounded with one or more additives such as pour point depressants, antioxidants, extreme pressure additives, and viscosity index (V.I.) improvers. As used herein, the term "stock," regardless whether or not the term is further qualified, will refer only to a hydrocarbon oil without additives. The term "solvent-refined stock" or "raffinate" will refer to an oil that has been solvent extracted, for example with furfural. The term "dewaxed stock" will refer to an oil which has been treated by any method to remove or otherwise convert the wax contained therein and thereby reduce its pour point. The term "waxy," as used herein, will refer to an oil of sufficient wax content to result in a pour point greater than -4 degrees C. (+25 degrees F.). The term "base stock" will refer to an oil refined to a point suitable for some particular end use, such as for preparing automotive oils.
The current practice for the preparation of high grade lubricating oil base stocks is to vacuum distill an atmospheric tower residuum from an appropriate crude oil as the first step. This step provides one or more raw stocks within the boiling range of about 288 degrees C. (550 degrees F.) to 565 degrees C. (1050 degrees F.) and a vacuum residuum. After preparation, each raw stock is extracted with a solvent, e.g. furfural, phenol or chlorex, which is selective for aromatic hydrocarbons, and which removes undesirable components. The vacuum residuum usually requires additional step to remove asphaltic material prior to solvent extraction. The raffinate from solvent refining is then catalytically dewaxed.
U.S. Pat. No. Re. 28,398 describes a process for catalytic dewaxing with a catalyst comprising zeolite ZSM-5. Such a process combined with catalytic hydrofinishing is described in U.S. Pat. No. 3,894,938. Catalyst dewaxing of raffinate using zeolite catalyst such as ZSM-5 is further described in U.S. Pat. Nos. 4,181,598 and 4,259,170, the entire contents of which are incorporated herein by reference.
The manufacture of lube oil base stocks is designed to produce a product according to very strict specifications for pour point, viscosity, viscosity index and product flash point. Often, improvements in one or more of these product parameters can be achieved only by adversely affecting other product parameters or at the expense of the overall yield of useful product. However, if improvements can be realized in the specification for viscosity index, for instance, without deleteriously affecting the achievable specification for the remaining product parameters very substantial economic benefits can accrue to the process of manufacturing of lube oil stock. If an improvement in the downstream processing of the lube oil cut from a crude oil can be improved to produce a lube oil stock with one or more properties exceeding the required specifications, the refinery operator is left with a variety of options on how to exploit the improved performance, all of which lead to a betterment of the economic performance of the refinery.
The art of lube base stock production is generally carried out by dewaxing raffinates by solvent dewaxing or catalytic dewaxing under conditions which produce a predetermined or target pour point for the base stock. Suitable materials are then added to the base stock to augment the base stock properties and meet the required specification, such as viscosity index. In catalytic dewaxing of raffinates to useful base stock having a predetermined pour point produced over medium pore zeolite catalysts, such as ZSM-5, viscosity indices (VIs) generally suffer substantial debits relative to solvent dewaxing.
It is an object of the present invention to provide an improvement to the process of catalytic dewaxing of raffinates for production of lube oil stock wherein the dewaxed product is produced at a much lower pour point under mild dewaxing conditions.
A further object of the present invention is to provide the aforenoted improvements while providing a base stock that exhibits a higher viscosity index (VI) rather than the reduced viscosity index typically found for lube produced by catalytic dewaxing and to achieve this object while maintaining or improving upon the overall product yield.